I think this says you need Professional Mode rather than Full Auto Mode.
Under File Save Settings in the Professional Mode, you should see buttons for Add Page / Edit Page / Safe File.
Basically, you keep adding pages then finish.
If you already have multiple .pdf files that you wish to combine, Acrobat Professional (not READER) can do that.
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I copied the text but I think you won't be able to see the images here:
4. Select PDF as the file type setting. The current Paper Size, Orientation, Margin, and Compression setting are displayed below it.
5. If you need to change any of the current PDF settings, click the Options button. You see the EPSON PDF Plug-in Settings window:
Select the settings that match your document and click OK. You return to the File Save Settings window.
6. When you are finished selecting file settings, click OK. Epson Scan scans your page and displays this window:
7. If you are scanning only one document page, or you don’t need to edit your pages, click Save File and go to step 10.
If you are scanning multiple pages, click Add page and replace the first page on the document table with the second page and click Scan. Repeat this step for each page in your document. When you are finished, click Edit page and go to step 8.
8. You see the Editing Page window displaying thumbnail images of each scanned page:
If you want to include all the document pages in your PDF file as they currently look, go to step 9.
If you want to delete some of the pages, click the odd pages or even pages icons, or click the individual pages you want to delete. Then click the delete icon to delete them.
If you want to rotate pages, click the pages or use the odd pages, even pages, or all pages icon to select the pages. Then click the left or right rotate icon to rotate them.
The delete icon is not available when all document pages are selected.
9. When you have finished editing your document pages, click OK. Your document is saved in a PDF file in the location you selected and you return to the EPSON Scan window.
10. Click Close to exit Epson Scan.